Arynes, chemistry reactivity

We hope that the results outlined in this review have demonstrated that the chemistry of the tetrahalogenobenzynes is sufficiently different from the chemistry of benzyne to be worthy of study. That four electron with drawing substituents are essential to the high reactivity of arynes in reactions with aromatic systems has been demonstrated by generating the isomeric trifluorobenzynes from the aryl-lithium compounds (135) and (136) in the presence of an excess of benzene 169>. Whereas tetra-fluorobenzyne reacts under similar conditions to give the 1,4-cycloadduct in greater than 50% yield the compounds (137) and (138) are produced in 16% and ca. 1.5% yields respectively. 

Reactive Intermediate Chemistry is an attempt to provide an updated survey and analysis of the field. We have adopted a three-dimensional approach. Reactive Intermediates are considered by type (e.g., carbocations, radicals, carbanions, car-benes, nitrenes, arynes, etc.) they are examined according to the kinetic realms that... 

The field of carbenes has been intensively investigated for the last two decades b. These studies have shown that carbenes as well as nitrenes and arynes play an important role in organic chemistry as reactive intermediates 2). Not only have these studies made it possible to understand many organic reactions in detail, they have also given use to a simple classification of organic reactions on the basis of these reactive intermediates. 

T. L. Gilchrist and C. W. Rees, Carbenes, Nitrenes, and Arynes, Chapter 8. Nelson, London, 1969 H. Heaney, Essays Chem. 1,95 (1970) N. S. Isaacs, Reactive Intermediates in Organic Chemistry, Chapter 7. Wiley, New York, 1974. 

There are many other kinds of reactive intermediates, which do not fit into the previous classifications. Some are simply compounds that are unstable for various possible reasons, such as structural strain or an unusual oxidation state, and are discussed in Chapter 7. This book is concerned with the chemistry of carbocations, carbanions, radicals, carbenes, nitrenes (the nitrogen analogs of carbenes), and miscellaneous intermediates such as arynes, ortho-quinone methides, zwitterions and dipoles, anti-aromatic systems, and tetrahedral intermediates. This is not the place to describe in detail the experimental basis on which the involvement of reactive intermediates in specific reactions has been estabhshed but it is appropriate to mention briefly the sort of evidence that has been found useful in this respect. Transition states have no real hfetime, and there are no physical techniques by which they can be directly characterized. Probably one of the most direct ways in which reactive intermediates can be inferred in a particular reaction is by a kinetic study. Trapping the intermediate with an appropriate reagent can also be very valuable, particularly if it can be shown that the same products are produced in the same ratios when the same postulated intermediate is formed from different precursors. 

For an introduction to the structures and reactions of carbenes, see (a) Liebman, J. F. Simons, J. in Liebman, J. F. Greenberg, A., Eds. Molecular Structure and Energetics, Volume 1 Chemical Bonding Models VCH Publishers Deerfield Beach, FL, 1986 p. 51 (b) Moss, R. A. Jones, M., Jr., in Jones, M., Jr. Moss, R. A., Eds. Reactive Intermediates, Vol. 2 John Wiley Sons New York, 1981 (c) Kirmse, W. Carbene Chemistry Academic Press New York, 1964 Gilchrist, T. L. Rees, C. W. Carbenes, Nitrenes and Arynes Appleton-Century-Crofts New York, 1969 (d) Mine, J. Divalent Carbon Ronald Press New York, 1964 Bertrand, G. in Moss, R. A. Platz, M. S. Jones, M., Jr., Eds. Reactive Intermediate Chemistry John Wiley Sons Hoboken, NJ, 2004 chapter 8 (e) Jones, M., Jr. Moss, R. A. in Moss, R. A. Platz, M. S. Jones, M., Jr., Eds. Reactive Intermediate Chemistry John Wiley Sons Hoboken, NJ, 2004 chapter 7. 

The strained nature of the aryne triple bond results in an easily accessible LUMO and so, arynes are excellent electrophiles that react with different anionic or neutral nucleophilic species. This low-lying LUMO also makes arynes prone to participate in a variety of pericyclic reactions. In addition, arynes are also able to undergo interesting transition metal-catalyzed processes. The increased availability of functionalized aryne precursors as well as the possibility of generating arynes under reagent-free conditions is allowing the development of new aryne-mediated synthetic strategies and the discovery of new reactivity patterns, which will expand the repertoire of chemistry that is actually possible with arynes. 

The chemistry of ort/w-arynes [106-109] has recently been undergoing a tremendous burst. Indeed, these versatile transient intermediates exhibit a unique reactivity, making them partners of choice for the functionalization of arenes by additions across the triple bond or for the synthesis of fused rings by pericychc reactions. 

Several reviews of the chemistry of strained cyclic alkynes and arynes and otiho-ztene cyclynes, " which include synthesis, structure, and reactivity of platinum complexes, have also appeared. The behavior of 1,4-enynes in the presence of transition metals has been reviewed. ... 

Since arynes and o-QDMs can be regarded as reactive equivalents of alkynes and 1,3-dienes, which have rich chemistry in catalytic transformations, there would be a number of unexploited reactions with arynes and o-QDMs, which further enhance the versatility of the dearomatization/aromatization sequence. 

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